Field of the Invention
This invention relates generally to pipeline rehabilitation methods and techniques in which an old underground pipeline is replaced, as by drawing a bursting head through the old pipe to burst it, while simultaneously pulling a new pipe through behind the bursting head. The invention has particular applicability to the rehabilitation of old asbestos-cement pipelines in which the burst up debris must be contained or encapsulated in some manner.
Description of the Prior Art
Asbestos-cement (AC) pipe was used extensively in the mid-1900s in potable water distribution systems, particularly in the western United States. During the post-World War II era, roughly 600,000 miles of asbestos-cement (AC) pipe was laid throughout the United States and Canada. These pipes—many of them up to 70 years old—have approached the end of their lifecycle and are corroding, cracking and breaking. The Chrysotile Institute estimates AC pipe lifespan at 70 years, but actual service life depends largely on pipe condition and working environment. Because thousands of miles of AC pipe installed in distribution systems in the U.S. is nearing the end of its useful service life, AC pipe condition assessment and strategic replacement planning, and project replacement implementation is necessary. Over time, AC pipe undergoes gradual degradation in the form of corrosion (i.e., internal calcium leaching due to conveyed water and/or external leaching due to groundwater). Such leaching leads to reduction in effective structural wall thickness and cross-section, which results in pipe softening and loss of mechanical strength. Accordingly, as the water distribution system ages, the number of AC pipe failures increases with time.
AC pipes were installed in the 1940s, 1950s and 1960s when asbestos cement was valued for its strength, durability and ease of use. In the 1970s, it was determined that asbestos was hazardous to humans who inhaled its fibers over long periods. The news media widely reported on the topic, showing footage of people in hazmat suits removing asbestos-containing materials from hospitals, offices, schools and other buildings. The U.S. Environmental Protection Agency (EPA) began regulating asbestos and limited its use. All of this contributed to a general unease among the public about the mineral, which has translated into a sense of unease by regulators about AC pipe replacement. This has complicated the issue of AC pipe renewal, even though science and common sense make it clear that AC replacement is safe when proper methods are employed and precautions taken. Still, regulatory bodies at the federal, state and local levels don't agree on how AC pipes can safely be replaced, EPA has deemed it safe to replace them, and has even said that the popular pipe-bursting method is permissible, but stops short of providing specific guidance on treating broken pieces of old AC pipe, which are considered a regulated asbestos-containing material (RAM) that is subject to National Emissions Standards for Hazardous Air Pollutants (NESHAP).
A survey conducted by the American Water Works Association (AWWA) in 2004 found that, on average, AC pipes constitute approximately 15-18 percent of the USA's water distribution and transmission systems. More detailed research indicates:
AC pipes comprised from 50-80% of typical storm drain systems in the western U. S. and Canada;
AC water transmission and distribution systems included a mileage range of 40-75% ACP in some USA cities;
AC pipe sewer systems include 10-25% AC pipe (mostly in force mains) in the USA.
The EPA has addressed replacement of AC pipe using, the pipe bursting method. In an advisory letter issued Jul. 17, 1991, the EPA stated its position that “the crushing of asbestos cement pipe with mechanical equipment would cause this material to become ‘regulated asbestos containing material’ (RACM)” and “ . . . the crushed asbestos cement pipe in place would cause these locations to be considered active waste disposal sites and therefore, subject to the requirements of §61.154 (NESHAP).” In this same letter, the EPA goes on to advise that “In order to avoid the creation of a waste disposal site which is subject to the Asbestos NESIIAP, the owners or operators of the pipe may want to consider other options for dealing with the abandoned pipe.” Since the EPA's letter did not specifically identify pipe bursting, interpretation of the intent is inconsistent throughout the industry.
Pipe bursting is a controlled process that takes place 3 to 5 ft. or more underground. It involves inserting a cone-shaped expander into the existing pipe to break it while simultaneously pulling through the new pipe in its place. As the new pipe enters, the old pipe crumbles gently and remains impacted in wet soil where it is contained and harmless. In addition to being a safe method of replacement, it's also one of the most cost-effective methods. City and urban residents like it because it can be done quickly with minimal disruption to the community and the environment. Pipe bursting is safe, because it all occurs underground where the pipe segments and debris can be confined.
The majority of pipe bursting done in the United States is done with pneumatic and static bursting tools. During pneumatic pipe bursting, the pipe bursting tool is guided through the deteriorated asbestos cement host pipe by a constant tension winch. As the tool travels through the pipe, its percussive action effectively breaks apart the old pipe and radially displaces and embeds the fragments into the surrounding native soil. Depending on the specific situation, the tool is equipped with an expander that radially displaces the host pipe fragments and makes room for the new pipe. During static pipe bursting, a larger tensile force is applied to the expander too via a pulling rod assembly or cable inserted through the existing pipe. The expander tool transfers the horizontal pulling force into a radial force—breaking the existing pipe and expanding the cavity. As the tool makes its way through the host pipe, it simultaneously pulls in the new pipe, usually HDPE or PVC or ductile-iron. Pipe bursting has the ability to leave an enlarged ‘hole’ for the trenchless installed pipe, thus providing an annular space between the expanded AC pipe, and its replacement pipe.
In the more detailed description of the invention which follows, pipe bursting is used as an exemplary technique for accomplishing the purposes of the invention. However, it will be understood by those skilled in the relevant arts that other techniques in addition to pipe bursting, such as pipe slitting or splitting or pipe fracturing may be employed, as well.